Given a string s, return the first non-repeating character in it and return its index. If it does not exist, return -1.
Too many methods to solve this question.
we can iterate each words in string's order. We can build a unordered_set. If this char is in the set, we skip current character. For each character, we iterate its successor and find the duplicates. If there is no duplicate, return current character.
AC!